If you're talking about the initial world (ground and sky) properties, the sky color, ambient light, and ground texture are all changeable.

If you're talking about the larger models which are used as settings (dojo, school, etc), you can import models that are formatted properly, but you can't form new models from scratch within Alice. You'd need a 3d modeling program like 3ds max or Blender.

Personally, I'd just fiddle with the different world properties until I got a unique combo I liked. It's easy, quick, and can be changed on a whim or even while the world is running.

Also, if you click on this.Ground in the Scene.setup, you will see the "Paint=" command set to the SurfaceAppearance that you selected. Click the Down Arrow, select "Import Image", and load a custom designed image.

Remember that this image will be repeated multiple times, so be careful that the edges match to some extent.

Try the quick .png file that I attached to see what it does to the ground. You should be able to right-click, and download it.

Unfortunately, my experiments discovered several problems when using Custom Images to skin objects, including most of the Shapes. The Dev Team has been informed, and hopefully it will be fully functional in the next release.